This invention relates to capped wheel nuts and, more particularly, to a method and apparatus for welding a cap to a wheel nut.
Capped wheel nuts for threading onto studs to secure automotive wheels to the vehicle axle, while at the same time protecting the otherwise exposed projecting end of the stud from environmental damage and providing it with an asthetic appearance, are well known. Such capped wheel nuts generally have a conventional nut body with a threaded opening, a conical end, which is intended to bear against a conical depression in a wheel, and wrench flats forming a hexagonal perimeter by which the nut may be applied and tightened on a stud. A sheath, generally of stainless steel, covers the wrench flats and encloses the end opposite to the conical end while leaving the threaded opening of the nut at the conical end exposed. Such sheaths are generally held on the nut body by various methods of attachment, including crimping against the nut body, adhesives and, as done in similar capped nut parts, by welding the cap to the nut.
An example of a welded capped wheel nut is shown in U.S. Pat. No. 4,123,961. A method and apparatus for forming a wheel nut with a welded cap is disclosed in U.S. Pat. No. 4,056,862. In the nut of that patent, the free end of a sheath which has been crimped against the section of the nut body that connects the end of the wrench flats to the conical wheel engaging surface is contacted, with pressured engagement, by a welding electrode. A second electrode is brought into pressured engagement with a section of the sheath that lies over a surface of the nut body to force the sheath area into intimate engagement with the nut body. An electric pulse, which may be produced by a bank of capacitors, having a duration of less than 30 microseconds and a peak current in excess of 50,000 ampheres is passed between the first and second electrodes. The second electrode is said to impose a force of at least 1,000 pounds per square inch between the sheath and the nut area adjacent to the sheath area contacted by the second electrode. A weld between the contacting surfaces at such adjacent areas is said to be thereby formed.
Thus, the method and apparatus disclosed in U.S. Pat. No. 4,056,862 requires that the two electrodes contact the nut and sheath and that the electrode contacting the sheath apply considerable force. Moreover, as shown in that patent, the method and apparatus form only one welded capped nut at a time which results in inefficiency, both in terms of reduced output per unit time as well as uneconomical use of welding energy. Since the use of welded capped nuts by automobile manufacturers is increasing in popularity, there exists a need for a method and apparatus which overcomes the deficiencies of the above described method and apparatus and provides advantages thereover.